The invention relates to a switching arrangement for telecommunication subscriber stations with key-operated selection units which drive central office line by switching in charge storages.
More particularly, the selection unit is a three terminal device which selects a series of surge currents, and is replaceable by other selection units. An impulse contact controlled by an electronic pulse generator circuit is formed by a semiconductor-controlled element, e.g., a transistor, and a bistable relay disables the speech circuit of the subscriber station throughout the duration of the dialing phase, and instead, maintains the transmitter and feeder circuit of which the selection unit is a part and is completed by depressing one of the keys of the selection unit.
An arrangement of the type described above is exemplified by the apparatus described in West German Offenlegungsschrift 2,210,852. In addition to its compatibility with other selection units because of the three point terminal, this arrangement is particularly advantageous because of its low power requirements, since the control of the bistable relay and of the impluse switch in the form of a semiconductor element requires only very little energy.
Bistable relays, however, are susceptible to shocks and may in fact be switched by vibrations. The use of such relays for switching from the speech phase to the dialing phase and vice versa thus requires additional steps so as to ensure that upon entry into service of the station the speech phase prevails. Such a solution is exemplified by West German Auslegeschrift 2,258,839, according to which additional cradle switches and additional terminals are needed for the selection unit, so that there is no compatibility with other prior art selection units.
A switching arrangement is known (cf. West German Pat. Application P 25,23,478.0) wherein the automatic return of the bistable relay after entry into service of the station is under the control of an integrated keysender module. However, this requires that the loop current flowing after entry into service of the station is adequate for the operation of the selection unit. Without the use of a voltage divider this requisite condition is not met if, for example, in the switching center high resistance line monitoring circuits are employed that do not respond upon switching of the selection unit.